Add asserts that the subvector size is a power of 2

This is an invariant needed if we want to check that the subvector exactly fills a vector register by checking if it's a multiple of vlen.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -2105,6 +2105,8 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
       bool ExactlyVecRegSized =
           Subtarget->expandVScale(SubVecVT.getSizeInBits())
               .isKnownMultipleOf(Subtarget->expandVScale(VecRegSize));
+      assert(isPowerOf2_64(Subtarget->expandVScale(SubVecVT.getSizeInBits())
+                               .getKnownMinValue()));
       assert(Idx == 0 && (ExactlyVecRegSized || V.isUndef()));
 #endif
     }

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -9858,6 +9858,8 @@ SDValue RISCVTargetLowering::lowerINSERT_SUBVECTOR(SDValue Op,
   }
 
   TypeSize VecRegSize = TypeSize::getScalable(RISCV::RVVBitsPerBlock);
+  assert(isPowerOf2_64(
+      Subtarget.expandVScale(SubVecVT.getSizeInBits()).getKnownMinValue()));
   bool ExactlyVecRegSized =
       Subtarget.expandVScale(SubVecVT.getSizeInBits())
           .isKnownMultipleOf(Subtarget.expandVScale(VecRegSize));